During the Founder's Day celebration, Jack, Allison, Fargo, Henry, and Jo somehow end up in 1947, in the middle of the army base that existed before Eureka was founded. They must find a way home before they change history or get caught.
Wil Wheaton guest stars as Dr. Isaac Parrish, an underappreciated scientist whose hostility appears to have gone viral. Blind rage is infecting everyone at Global Dynamics with potentially catastrophic results -- especially for Fargo.
Claudia Donovan pays a visit to Eureka, hoping to see some amazing technological wonders. She gets more than she bargained for when seemingly random objects begin appearing around town. Working with Sheriff Carter and Fargo, Claudia has to help solve the mystery before the strange anomalies have deadly consequences...particularly for newcomer Dr. Grant.
Allison accepts Carter's offer to include Kevin in a camping trip for some guy time, and Henry decides to level with Grace while a very peculiar -- and dangerous -- relationship threatens Eureka and the not-so-happy campers.
Romantic relationships are in flux. Zoe is home for a visit and hints at a new heartthrob. Henry is courting his wife while Dr. Grant and Carter spar over Allison. Jo, on the other hand, is done with love for now and just wants to get her house rebuilt. But that project is on hold because hunky paleo-anthropologist Dr. Derek Thurgood says he found ancient human remains at the site and refuses to allow the build to continue.
Mid-Season Finale! Closer than they've ever been before, Allison (Salli Richardson-Whitfield) and Carter (Colin Ferguson) join forces personally as well as professionally when Jo (Erica Cerra) and Fargo (Neil Grayston) enlist them to help find a stolen Global Dynamics project.
Sheriff Carter relates a Christmas story in which the defensive field malfunctions trapping everyone in town for the holiday. So Fargo throws a big party, unaware that the town is shrinking. All from a quest for the perfect fruitcake.
When the microbots assembling the Titan space craft finish their task they opt to take on a new project and proceed to devour Eureka in the process. In isolation for a mission evaluation the Titan team can't tell if the microbot catastrophe is real or just part of the test.
Preparations for the Titan mission have the unintended consequence of turning the local Eureka environment into Titan's. Meanwhile, Eureka couples struggle to come to terms with each other and the DoD sends an investigator to evaluate Jack and Allison's relationship.
Sheriff Carter and Allison Blake are secretly planning the perfect holiday surprise for their kids, but a mysterious kaleidoscopic wave of color crashes over Eureka, leaving the entire town and its inhabitants animated.